The present invention relates to integrated circuit operational amplifiers (op-amps) having high precision and a "rail-to-rail" common mode input range. More particularly, for op-amps having two differential input stages, methods and circuitry are provided for trimming the input stages in order to minimize input offset voltage.
The op-amp is a basic building block in the design of many analog electronic circuits. Because commercial op-amp devices are inexpensive, easy to use, and widely available, they can be found in a wide range of electronic devices, from medical instruments to consumer electronics.
Commercial op-amp devices are typically subject to one or more non-ideal performance limitations. For example, actual op-amps have finite values for voltage gain, input impedance, and bandwidth, as well as a non-zero output impedance. In addition, tolerances in manufacturing op-amps may result in other non-ideal characteristics, such as the presence of an input offset voltage, which is discussed further below. Many op-amp circuits have been designed to make their performance closer to that of the ideal op-amp. Frequently performance is improved in one area at the expense of decreased performance in another area.
An op-amp of conventional design has an input stage consisting of a pair of transistors configured as a differential amplifier having two symmetrical circuit branches, wherein each branch includes a transistor coupled to one of the input terminals. In each branch, the transistor produces a signal proportional to the voltage on the corresponding input terminal. The output of the input stage is the difference between the signal in each branch of the differential amplifier. Ideally, the values of corresponding circuit components in the two branches, and hence the branches themselves, are identical, so that when identical voltages are applied to each input, i.e. a common-mode input voltage, the signals in each branch are also identical, and the output of the input stage is zero.
In actuality, the branches are not exactly identical. Manufacturing tolerances can cause the values of circuit components in one branch of the input stage to differ slightly from the values of the corresponding component in the other branch. Because of this lack of symmetry, the branches of the differential amplifier have slightly different characteristics, causing a common-mode input voltage to result in nonequal signals in each branch of the differential amplifier. Therefore the output signal, which corresponds to the difference between the two branch signals, may be non-zero for a common-mode input signal. To compensate for a non-zero output resulting from a common-mode input a small voltage difference can be applied between the input voltages to produce a zero output voltage. The small voltage difference that would be required to produce an output voltage of zero is called the `input offset voltage,` or just `input offset,` and is an electrical characteristic of an op-amp.
While the input offset varies from op-amp chip to the next, a particular op-amp design has associated with it characteristic values of input offset voltage, such as typical and/or maximum values. An op-amp design which exhibits lower characteristic values of input offset voltages than another design is said to be more precise, or to have more precision, than the other design. Manufacturer's frequently include the characteristic values of input offset voltage on an op-amp's specification sheet.
The presence of an input offset voltage in an op-amp introduces a DC offset into the output signal. In some applications, such as designing instrumentation amplifiers, it is necessary to be able to amplify signals having a very small magnitude; particularly, with respect to such applications, reducing the output DC offset by minimizing the input offset voltage of an op-amp is a desirable goal in op-amp design. It is possible using previously known techniques to trim the values of various components in the input stage circuitry, usually resistive elements, in an attempt to restore symmetry to the branches of the differential amplifier and thereby minimize the magnitude of the input offset voltage.
Another characteristic of op-amps of conventional design is the limited range of common-mode input voltages. A common-mode input voltage which is at or near one of the supply voltages (+V.sub.CC and -V.sub.EE) may drive the transistors in the input stage into either a saturation or cutoff condition. This limits the useful range of common-mode input voltages since they must not approach or exceed either of the op-amp's supply voltages. A conventional rule of thumb is that the input signal should not come within about 1 volt of either +V.sub.CC or -V.sub.EE.
For many applications, it is desirable to allow the input signal of an op-amp to have a common-mode input voltage range which includes both +V.sub.CC and -V.sub.EE. This can be accomplished by utilizing an input stage having two complementary differential amplifiers coupled in parallel, wherein one of the differential amplifiers is active for input signals at or near +V.sub.CC, and the other differential amplifier is active for input signals at or near -V.sub.EE. The outputs of the two differential amplifiers are then summed to obtain an output for the input stage. For input signals that are not near either supply voltage, both of the differential amplifiers are active to a varying degree. Additional circuitry may be incorporated to provide for a smooth transition between states in which only one or the other of the differential amplifiers is active as a common-mode input voltage varies from one supply voltage to the other. In this way, the common-mode input range is extended to include both power supply voltages.
However, the increased complexity of an op-amp having dual differential amplifiers in the input stage, potentially increases the number of errors during the fabrication process. Just as variations in symmetry between branches of a single differential input stage cause an op-amps of conventional design to exhibit a characteristic input offset voltage, an operational amplifier design employing dual input stage differential amplifiers is subject to similar variations in the symmetry between branches of each of its differential amplifiers. Accordingly, an op-amp having dual differential amplifiers typically has an input offset voltage that depends upon which of the input stages is active. Thus, while dual input stage differential amplifiers have been used to successfully extend the common-mode input range in the prior art, previously known realizations have been untrimmed and therefore lack precision.
In view of the foregoing, it would be desirable to provide an op-amp having a common-mode input range which includes the voltage on both supply rails, and which has low characteristic input offset voltages.
It would also be desirable to provide circuitry and methods for improving the precision of an op-amp having dual input stage differential amplifiers.
In addition, it would be desirable to be able to trim both differential amplifiers of an op-amp having dual input stage differential amplifiers in a manner that allows the effect of trimming to be closely approximated using conventional analytical techniques.